The objective of this project is to use monoclonal antibodies to identify and characterize specific cell surface molecules of murine hematopoietic cells. During the past year we have continued our program of producing and characterizing new monoclonal antibodies. We have immunized rats with murine hematopoietic cells or tumors of these cells, hybridized spleen cells from these animals to the nonproducer murine myeloma S194/5.XXO.BU.1, and screened supernatants from the resulting hybridomas by indirect binding and, subsequently, by immunoprecipitation. In this way, we have identified antibodies defining molecules which have so far not been described on hematopoietic cells. Over the past year, we have concentrated on three cell surface molecules: (1)\ThB. This molecule is defined by monoclonal antibody RGRSL 114.8.1 which precipitates a molecule with MW approximately 11,000 daltons. (2)\Pgp-1 glycoprotein. We have isolated a number of monoclonal antibodies which precipitate a molecule with MW approximately 95,000. This molecule is widely distributed on both hematopoietic and nonhematopoietic tissues and is a major cell surface glycoprotein of myeloid cells. (3)\Murine transferrin receptor. We have identified two monoclonal antibodies which recognize the murine transferrin receptor. Our objectives for the coming year are: (1)\Continue to obtain monoclonal antibodies against normal hematopoietic cells and their tumors. Characterize these antibodies to identify those detecting previously undefined molecules and/or showing a restricted tissue distribution. Determine distribution on normal tissues and behavior in functional assays for stem and progenitor cells. (2)\Continue studies on nature of Pgp-1 positive cells in the thymus. Determine ability of Pgp-1 positive cells to recolonize the thymus in in vivo reconstitution assays. Characterize the nature of the Pgp-1 positive cell in the thymus (size, cell surface antigen patterns) and attempt to establish cell lines from these cells. (3)\Use in vitro and in vivo functional assays to characterize stem and progenitor cells for the murine transferrin receptor. Attempt to obtain mutants of Friend virus-induced erythroleukemias with defects in expression of transferrin receptor expression; study transferrin receptor expression in noninducible variants of Friend virusinduced erythroleukemias.